Silk is a natural protein fiber, some forms of which can be woven into textiles. The best-known type of silk is obtained from cocoons made by the larvae of the silkworm Bombyx mori reared in captivity (sericulture). The shimmering appearance for which silk is prized comes from the fibers' triangular prism-like structure which allows silk cloth to refract incoming light at different angles.
"Wild silks" or tussah silks (also spelled "tasar") are produced by caterpillars other than the mulberry silkworm (Bombyx mori). They are called "wild" as the silkworms cannot be artificially cultivated like Bombyx mori. A variety of wild silks have been known and used in China, India, and Europe from early times, although the scale of production has always been far smaller than that of cultivated silks. Aside from differences in colors and textures, they all differ in one major aspect from the domesticated varieties: the cocoons that are gathered in the wild have usually already been damaged by the emerging moth before the cocoons are gathered, and thus the single thread that makes up the cocoon has been torn into shorter lengths. Commercially reared silkworm pupae are killed before the adult moths emerge by dipping them in boiling water or piercing them with a needle, thus allowing the whole cocoon to be unraveled as one continuous thread. This allows a much stronger cloth to be woven from the silk. Wild silks also tend to be more difficult to dye than silk from the cultivated silkworm.
There is some evidence that small quantities of wild silk were already being produced in the Mediterranean area and the Middle East by the time the superior, and stronger, cultivated silk from China began to be imported.
Silks are produced by several other insects, but only the silk of moth caterpillars has been used for textile manufacture. There has been some research into other silks, which have differences at the molecular level. Silks are mainly produced by the larvae of insects with complete metamorphosis, but also by some adult insects such as webspinners. Silk production is especially common in the Hymenoptera (bees, wasps, and ants), and is sometimes used in nest construction. Other types of arthropod produce silk, most notably various arachnids such as spiders.
India and Nepal
Silk, known as Pattu or Reshmi in southern parts of India and Resham in Hindi, has a long history in India and is widely produced today. Historically silk was used by the upper classes, while cotton was used by the poorer classes. Today silk is mainly used in Bhoodhan Pochampally (also known as Silk City), Kanchipuram, Dharmavaram, Mysore, etc. in South India and Banaras in the North for manufacturing garments and Sarees. "Murshidabad silk", famous from historical times, is mainly produced in Malda and Murshidabad district of West Bengal and woven with hand looms in Birbhum and Murshidabad district. Another place famous for production of silk is Bhagalpur. The silk from Kanchi is particularly well-known for its classic designs and enduring quality. The silk is traditionally hand-woven and hand-dyed and usually also has silver threads woven into the cloth. Most of this silk is used to make saris. The saris usually are very expensive and vibrant in color. Garments made from silk form an integral part of Indian weddings and other celebrations. In the northeastern state of Assam, three different types of silk are produced, collectively called Assam silk: Muga, Eri and Pat silk. Muga, the golden silk, and Eri are produced by silkworms that are native only to Assam. The heritage of silk rearing and weaving is very old and continues today especially with the production of Muga and Pat riha and mekhela chador, the three-piece silk saris woven with traditional motifs. Mysore Silk Sarees, which are known for their soft texture and expensive class last easily as long as 25 to 30 years, if maintained well.
Major Fiber Properties
- Shape:Silk has a triangular shaped cross section whose corners are rounded.
- Luster:Due to the triangular shape (allowing light to hit it at many different angles), silk is a bright fiber meaning it has a natural shine to it.
- Covering Power:Silk fibers have poor covering power. This is caused by their thin filament form.
- Hand:When held, silk has a smooth, soft texture that, unlike many synthetic fibers, is not slippery.
- Denier:4.5 g/d (dry) ; 2.8-4.0 g/d (wet)
- Strength:Silk is the strongest of all the natural fibers; however it does lose up to 20% of its strength when wet.
- Elongation/elasticity:Silk has moderate to poor elasticity. If elongated even a small amount the fibers will remain stretched.
- Resiliency:Silk has moderate wrinkle resistance
- Protein Composition:Silk is made up of GLY-SER-GLY-ALA-GLY and forms Beta pleated sheets. Interchain H-bonds are formed while side chains are above and below the plane of the H-bond network. Small residue(Gly) allows tight packing and the fibers are strong and resistant to stretching. The tension is due to covalent peptide bonds. Since the protein forms a Beta sheet, when stretched the force is applied to these strong bonds and they do not break. The 50% GLy composition means that Gly exists regularly at every other position.
- Absorbency:Silk has a good moisture regain of 11%.
- Electrical Conductivity:Silk is a poor conductor of electricity making it comfortable to wear in cool weather. This also means however, that silk is susceptible to static cling.
- Resistance to Ultraviolet Light/Biological Organisms:Silk can become weakened if exposed to too much sunlight. Silk may also be attacked by insects, especially if left dirty.
- Chemical Reactivity/ Resistance:Silk is resistant to mineral acids. It is yellowed by perspiration and will dissolve in sulphuric acid.
Dimensional Stability:Unwashed silk chiffon may shrink up to 8% due to a relaxation of the fiber macrostructure. So silk should either be prewashed prior to garment construction, or dry cleaned. However, dry cleaning may still shrink the chiffon up to 4%. Occasionally, this shrinkage can be reversed by a gentle steaming with a press cloth. Gradual shrinkage is virtually nonexistent, as is shrinkage due to molecular-level deformation.
Uses for Silk
Apparel:Silk is excellent for use in warm weather and active clothing. The silk's good absorbency makes it comfortable to wear in such conditions. Silk is also excellent in the cold because its low conductivity keeps the wearer warm.
Examples of Silk Clothing
- Formal Dresses
- High Fashion
Silk's elegant, soft luster and beautiful drape makes it perfect for many furnishing applications.
Examples of Silk Furnishings
- Wall Coverings
- Window Treatments (if blended with another fiber)
- Wall Hangings
As the process of harvesting the silk from the cocoon kills the larvae, silk-culture has been criticized in the early 21st century by animal rights activists on the grounds that silk production kills silkworms, and that artificial silks are available. Others point out that silkworms depend upon humans for their survival, and would become extinct without humans to care for the worms and harvest the silk.
Mahatma Gandhi was critical of silk production based on the Ahimsa philosophy. Ahimsa is part of the three millennial Jain philosophy of India "not to hurt any living thing," which led to development of a cotton spinning machine he distributed. Such a machine can be seen in the Gandhi Institute.
Ahimsa Silk, made from the cocoons of wild and semi-wild silk moths, is being promoted in parts of Southern India, for those who prefer not to wear silk produced which involves the death of silkworms.
Mongols used silk as part of the under-armor garments. Silk is so tough that it was actually used as very light armor, although its special use was to stop arrow penetration into the body. The silk would stop an arrow from penetrating far enough into the body to be lethal, and the arrow could be pulled out of the wound by tugging on the unbroken silk. The head of an arrow pulled out this way would not contact the body, reducing the likelihood of infection.
In addition to clothing manufacture and other handicrafts, silk is also used for items like parachutes, bicycle tires, comforter filling and artillery gunpowder bags. Early bulletproof vests were also made from silk in the era of blackpowder weapons until roughly World War I. Silk undergoes a special manufacturing process to make it suitable for use as non-absorbable surgical sutures. Chinese doctors have also used it to make prosthetic arteries. Silk cloth is also used as a material to write on.
- Silk moths lay eggs on specially prepared paper.
- Eggs hatch and the caterpillars are fed fresh mulberry leaves.
- After about 35 days, and 4 moltings, the silkworms are 10,000 times heavier than when hatched – they are now ready to begin spinning a cocoon.
- A straw frame is placed over tray of silkworms – they begin spinning cocoons by moving their heads in a figure 8.
- Liquid silk, coated in sericin, is produced in 2 of the silkworm’s glands, which is forced through spinnerets.
- Sericin: water-soluble protective gum
- Spinnerets: openings in silkworm’s head
- As this liquid silk comes into contact with the air, it solidifies.
- Within 2-3 days, the silkworm will have spun 1 mile of filament and will be completely encased in a cocoon.
- After this entire process, the silkworm metamorphoses into a moth, but is usually killed by heat before it reaches the moth stage – any silkworm reaching the moth stage is used for breeding the next generation of silkworms.
- Cocoons that have been stifled are sorted by fiber size, fiber quality, and defects, then are brushed to find filaments.
- Several filaments are gathered together and wound onto a wheel (reeling).
- Each cocoon yields 1000 yards of silk filament, known as raw silk, or silk-in-the gum, fiber.
- Several filaments are combined to form a yarn.
- As fibers are combined and wrapped into the reel, twist can be added to hold the filaments together.
- Adding twist is referred to as ‘throwing’ – resulting yarn is called thrown yarn
- The type of yarn and amount of twist relate to the fabric produced.
- The simplest type of thrown yarn is a ‘single’ – 8 filaments are twisted together to form a yarn
- Used for filling yarns in silk fabrics, singles can have 2 or 3 twists per inch
Wild silk production is not controlled. Cocoons are harvested after the moth has matured, so silk cannot be reeled – it must be spun.
Types of Wild Silk
- Tussah (most common)
- Momme (standard way to describe silk fabrics)
With over 30 countries producing silk the major producers are:
- China (54%)
- India (14%)
- Japan (11%)